As well known, in conventional gas turbines, acoustic oscillation usually occurs in the combustion chambers of the gas turbines. With the term chamber is intended any gas volume where combustion dynamics occur. In such chambers the flow of a gas (for example a mixture of fuel and air or exhaust gas) with high velocity usually creates noise. Burning air and fuel in the combustion chamber causes further noise. This acoustic oscillation may evolve into highly pronounced resonance. Such oscillation, which is also known as combustion chamber pulsations, can reach amplitudes and associated pressure fluctuations that subject the combustion chamber itself to severe mechanical loads that may decisively reduce the life of the combustion chamber and, in the worst case, may even lead to its destruction.
To reduce the acoustic oscillations noise it is well known in the art to install acoustic damping devices like Helmholtz resonators.
Typically, these kinds of dampers are physical devices that are often positioned around the combustion chamber (on the liner, on the front panel). They usually include an empty cavity (where air can flow) and a neck that connects the volume of the cavity to the combustion chamber.
The resonance frequency and damping power of a Helmholtz damper assembly depends on its geometry and on the flow through its neck.
Once the Helmholtz damper is selected and its geometry fixed, it provides a specific characteristic to damp certain frequencies with a certain growth rate reduction coefficient. According to the teachings of the prior art, the geometry cannot be changed during rig or engine operation.
To change the frequency, or to deactivate a damper assembly, the rig/engine has to be shut off and partly disassembled. However, it will be appreciated that such procedure is time-consuming and during following test run only one configuration can be tested.
Moreover, in the event that a wrong arrangement is chosen, the following test is useless or even an outage has to be repeated. To reduce the risk of such outages and/or unsuccessful tests, normally several damper assemblies are connected to the combustion chamber. Such methodology might eventually lead to engines having a large number of dampers.
In sum, up to now different damping frequencies are achieved with several damper assemblies. Such damper assemblies are always active whether they are needed or not for a specific operation regime (e.g. gas or oil operation or part or full load). If certain damper assemblies would not be needed during full load, purge air would still cool down the combustor chamber and increase NOx.